I have always been curious but never enough to ask this.I have a Colt pre-ban HBAR purchased new by me in '86.The receiver is marked AR-15 A2 HBAR Sporter cal. .223 Ser. SP21XXXX.Now, I read here and elsewhere, where four digit model numbers are used to describe such rifles. Owners/shooter rattle these numbers off like their DOB's.Could someone please tell me the correct model # designation for my rifle?